The present invention relates to melt processible fluoropolymer compositions made by blending at least two different melt processible tetrafluoroethylene copolymers.
Tetrafluoroethylene-hexafluoropropylene copolymers (also known as TFE/HFP) have the physical, chemical, mechanical, thermal and electrical properties characteristic of fluoropolymers and can be easily extruded or injection molded. HFP content of commercial copolymers is within the range 9.6 to 17.6 wt % (Ullmann""s Encyclopedia of Industrial Chemistry, 5th ed., vol. A 11, p. 403, xc2xa72.3.2, VCH Verlagsgesellschaft mbH, D-6940 Weinheim, Germany, 1988. The HFP index referred to in this article can be converted to wt % HFP by multiplying by 3.2, as disclosed in U.S. Pat. No. 5,700,889.). Such properties have led to a growth in the demand for TFE/HFP copolymers, particularly in wire coating applications (e.g., plenum cable). To further increase productivity, there is a need for tetrafluoroethylene-hexafluoropropylene copolymers that can be extruded at higher speeds without loss of extrudate continuity or reduction in the mechanical properties of the extruded article, for example fluoropolymer-coated wire. Existing TFE/HFP copolymers are unable to fully meet to such requirements. Although the melt flow of TFE/HFP can be increased by lowering its molecular weight, this is accompanied by a decline in the mechanical properties of the shaped article and a reduction in melt strength, i.e. a tendency for the molten polymer to tear or separate frequently while being stretched and coated, for example onto a metal conductor. This tearing limits the speed of the coating operation. Thus, lowering the molecular weight does not constitute a basic solution.
A number of cases are known in which fluoropolymers have been blended to improve the physical properties of the melt processible fluoropolymer. For example, blends of TFE/HFP and tetrafluoroethylene-perfluoro(alkyl vinyl ether) (PFA) are described in U.S. Pat. No. 5,041,500, and in the Journal of Polymer Science: Polymer Physics 37, p. 679 (1999). The patent discloses mixtures of TFE/HFP copolymers with tetrafluoroethylene-perfluoro(propyl vinyl ether) (TFE/PPVE), but states that the two components do not co-crystallize. The above-referenced article describes a miscible blend of a TFE/HFP with tetrafluoroethylene-perfluoro(methyl vinyl ether) (TFE/PMVE). However, the TFE/HFP has a low comonomer content and is therefore very similar to polytetrafluoroethylene (PTFE), making it unattractive as a melt-processible material.
European Patent No. 1 000 976 A1 discloses blends of crystalline fluoropolymer with amorphous fluoropolymer or with fluoropolymer containing amorphous segments and crystalline segments for the purpose of reducing the size of the spherulites in the polymer. These segmented copolymers are also known as block polymers because the distribution of monomers throughout the polymer varies according to the block or segment of the polymer that is being considered. In contrast, typical commercial fluoropolymers are xe2x80x9crandomxe2x80x9d or xe2x80x9cstatisticalxe2x80x9d copolymers, the distribution of the monomers along the polymer chain being determined by the monomer reactivity ratios and the concentrations of monomers during the polymerization. Other things being equal, amorphous fluoropolymers are more costly and difficult to make than crystalline fluoropolymers because the former contain less of the lower cost and more reactive fluoromonomer, tetrafluoroethylene. Block copolymers are also more difficult to make because they are conventionally prepared in multistep processes.
New blends of fluoropolymers are needed that will permit faster extrusion rates with no sacrifice of melt strength or mechanical properties, and preferably with improved melt strength and mechanical properties.
In accordance with the present invention, improvements in TFE/HFP copolymers are obtained by blending therein a PFA based on perfluoro(methyl vinyl ether) (PMVE) or perfluoro(ethyl vinyl ether) (PEVE). In particular it has been discovered that the use of a TFE/HFP dipolymer of a specific composition, or a TFE/HFP terpolymer of a specific composition arrived at by copolymerizing a small amount of PPVE or PEVE or PMVE with the TFE and HFP, yields a blend with the PFA copolymer having good miscibility. The resulting blend has a flex life and melt strength that are greatly improved over those of TFE/HFP alone. The blend also has improved high-speed processibility in wire coating applications.
Thus the present invention is a melt processible fluoropolymer composition that is a miscible blend of at least two different melt processible fluoropolymers, which composition partakes of the excellent properties of each of the starting materials, and also has improved high-speed wire coating processibility and flex life. By miscible is meant that the composition is homogeneous in the melt phase and that the components co-crystallize on cooling to the solid phase.
Accordingly, the invention provides a melt processible fluoropolymer composition having a single crystallization temperature and a single melting point, as measured with a differential scanning calorimeter, said composition comprising:
(A) about 3 to about 97 parts by weight of a tetrafluoroethylene-hexafluoropropylene copolymer comprising about 3 to about 9% by weight (wt %) hexafluoropropylene, and 0 to about 4 wt % of a third monomer that is at least one additional monomer selected from the group consisting of perfluoro(propyl vinyl ether) (PPVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether) (PMVE), and about 89 to about 97 wt % tetrafluoroethylene; and
(B) about 97 to about 3 parts by weight of a copolymer comprised of about 80 to about 99 wt % tetrafluoroethylene with about 1 to about 20 wt % of one or more comonomers selected from the group consisting of perfluoro(ethyl vinyl ether) and perfluoro(methyl vinyl ether).